request: allow fix bond/break to change atom types

Suppose I want to modify "fix_bond_break.cpp" so that whenever a bond
is broken, both atoms from the bond are also changed to atom type 2
(for example). How would I go about this?

--- Goal ----
Actually, I have already written a much more general fix (based on
"fix_bond_break.cpp") which allows LAMMPS users to simulate
"intelligent molecules". These molecules can make or break bonds and
make arbitrarily complex decisions to alter their properties in
response to contact with other molecules. (Essentially, this fix
allows LAMMPS users to run arbitrary cellular automaton on networks of
bonded atoms. There's a Conway's game of life example which uses this

The reason we created this fix was to study coarse-grained machines in
cells which are driven out of equilibrium and exhibit complex behavior
(such as microtubules which are dynamically unstable, as well as
proteins which read and walk along DNA controlling gene expression and
cell fate). Because the fix is general, the hope is that anybody
could use it to model new biomolecules we don't understand yet. We
want to eventually get many biologists interested in using LAMMPS.

Alas, I have not released this fix because when I run it in parallel,
it crashes, (unsurprisingly). But it only seems to crash when I ask
it to alter the atom types. Rather than post that code, just knowing
the answer to the question above would be incredibly useful.

Thanks to everyone who read this far.

P.S. We are currently writing a paper on this, in case anyone wants be
part of that. Help on this issue would definitely earn authorship.

P.P.S. More specific questions (feel free to ignore these):
1) I'm curious: Is it a problem to change the atom types from within
FixBondBreak::post_integrate()? (I ask because I noticed that
"fix_atom_swap.cpp" places the code that changes the atom types inside
"pre_exchange()" instead)
2) Is there anything else I have to do after changing the atom types
besides invoke comm->forward_comm_fix() and update
"un/pack_forward_comm()"? (To make it easier, let's assume, for the
moment, that the distance c.utoff is the same for all atoms types.
I'll worry about that issue later.)

Hi Andrew - you could look at fix bond/create, which

has some logic to change an atom type under certain


In principle changing atom types for atoms in a bond

should be OK to do. You need to insure that all other

procs know about the change, e.g in their ghost or owned

atoms. The forward comm should accomplish that

for ghost atoms. If you had one proc change the

type of a ghost atom owned by another proc, then

you’d need a reverse comm as well, to propagate that.

Re: bigger picture, you should talk to Jake Gissinger, CCd.

He’s been working on a general “fix react” command to

allow more complex reactions than fix bond break/create allow,

and your model might fit into that framework.


Thanks for the prompt reply. Detailed comments below…

Thanks? That’s very reassuring to hear.


  1. Is it possible to forgo all this simply by invoking “neighbor->build()” at the end of the fix’s “post_integrate()”. (I can worry about optimizing speed later.).

  2. If so, then, after modifying the contents of the local atom->type[] array, would I need to do anything else before doing this?

Interestingly, un/pack_forward_comm() from “fix_bond_create.cpp” does not pack or unpack atom type information (just partner lists, probabilities, and special bond information).
3) (I’m curious, how does it transmit atom type changes to other procs?)

Good point.

I will have to modify both atoms in the bond, and one of them could be owned by another processor. Unfortunately neither fix_atom_swap.cpp, nor fix_bond_create.cpp can do this.

  1. Actually, I was thinking of reporting this as a bug in fix_bond_create.cpp: Users can specify either “iparam” OR “jparam”, but not both. (If they specify both, then only one of the atoms will be changed.) This significantly limits what “fix bond/create” can do, and makes it makes it practically impossible to use it for many polymerization problems (for example).

(I also have a selfish motive to complain about this: If someone solved this issue, then I could probably steal that code and put it in the fix I am working on. I might post a separate question about this on the list…)

Either way, I am interested to learn of any other fixes I can study which can modify properties of atoms belonging to other processors.

Thanks for engaging me in a discussion on this topic. I think it is really important (and not just for my own work). LAMMPS already has fantastic capabilities and infrastructure. With some really trivial changes like this, LAMMPS will be able to do exiting, amazing new things.

I’m CCing this to Tim Sirk who was also modifying trying to modify fix_bond_create.cpp. (Hi Tim)

Bad idea to try to invoke an extra neighbor rebuild from inside your fix.

Instead, set the next_reneighbor flag (see fix.h) to trigger a neighbor

rebuild in the normal timestep. That comes immeditately after

post_intergrate() anyway. That’s what fix bond/create does.

Re: limitations of fix bond create/delete. Again I suggest conferring

with Tim and Jake - they have been working on a bigger picture

solution to these kinds of issues.


Bad idea to try to invoke an extra neighbor rebuild from inside your fix.
Instead, set the next_reneighbor flag (see fix.h) to trigger a neighbor
rebuild in the normal timestep. That comes immeditately after
post_intergrate() anyway. That's what fix bond/create does.

Dear Steve, Jacob, and Tim

   Thanks for making code suggestions and sending me example code.
I'll try Jacob's version get back to you if it works. (I tried
something similar to Jacob's solution without success, but my version
did other things was much more complicated. My fault. I should
always start with something simple.)

   I've been struggling with perplexing crashes since I started toying
around with these kinds of fixes a week ago. I probably should not
have attempted it. Forgive my impatience. I've got a hard deadline
and I'm not supposed to be working on this stuff at all. So it's
fantastic to have examples of code that does something close to what
I'm trying to do. I really appreciate it.


P.S. If I use any of your code, I will contact you before any paper
submission that use it are made. Perhaps we can also work together to
make a version of fix bond create/break which can handle all of the
goals that we have, or at least share as much code as possible.
(Jacob, have you had a look at Tim's "topo.cpp",
"fix_bond_create_rxn.cpp", and "fix_bond_break_rxn.cpp" files?)

Here are some things to think about in the future:

1) There are some cases where it's very convenient for the changes
made by one fix need to be finalized before the next one begins,
(without any atom movement in between, for example to prevent an atom
from drifting too far after a bond break). When this happens, is
there a way to tell LAMMPS that you don't want to move any atom
positions for the next N verlet iterations? (Where "N" is the time
interval between when the fix is applied)

2) Minimization following topology changes:
    It would be great if there was a way to define a dynamic group of
atoms which were "recently_influenced" by the formation or deletion of
nearby bonds, and send only that group of atoms to be minimized.
(Perhaps this could be done using a for-loop that run dynamics for N
steps, and then invokes "minimize" combined with "setforce" to
immobilize atoms which aren't in the "recently_influenced" group).
The sudden changes in energy caused by creating a new angle
interaction can be surprisingly large. An alternative approach might
be to slow them down temporarily by changing their mass or friction
coefficients for the next N verlet steps.

(I like to pontificate.)